textcomp only provides additional symbols, for example, math symbols (arrows, delimiters), currency symbols, diacritics, and greek symbols that can be used in text mode. I would use it by default, just in case. The author of siunitx (Joseph, he is a moderator here on latex.org btw.) would prefer textcomp too, but he did not want to enforce users to load it so he provides some symbols, such as the micro symbol, himself.